To provide a reinforced fiber sheet suitable for e.g. repairing concrete surface cracks.

The reinforcing fiber sheet S1 comprises reinforcing fibers composed of warps 1 with a tensile strength of less than 800 MPa and wefts 2 with a tensile strength of 800 MPa or greater. A method for reinforcing cloth foundation uses the reinforced fiber sheet S1 applied on the concrete surface so that the direction of the warps 1 nearly coincide with that of a crack C. Thus, the reinforced fibers (i.e. the wefts 2) is applied in a direction nearly rectangular to the crack C (the direction in which tensile stress acts) to suppress the propagation of cracks C against the tensile stress .